The present invention relates to a dryer section of a paper machine in which so-called normal drying groups having a single-wire draw are provided with heated contact-drying cylinders arranged in an upper row and leading cylinders or rolls arranged in a lower row. At least one drying module in accordance with the present invention is arranged between adjacent normal drying groups and/or inside the normal drying groups in the dryer section between a pair of drying cylinders. In the drying module, the side of the web which is not placed against the contact-drying cylinders in the normal drying groups, i.e., an opposite side of the web, is placed against the heated cylinder face of a large drying cylinder. The present invention also relates to a method for drying a paper web.
The highest web speeds in paper machines are currently already in a range of about 25 meters per second. In the near future, the web speeds which will be used in paper machines will be in a range from about 25 m/s to about 40 m/s. With the current highest running speeds of webs and with even higher future running speeds planned, the dryer section in particular has become, and will be, the bottleneck of the runnability of a paper machine, i.e., it will impede the functionality of the advantageously higher web running speeds. Thus, it is desirable to develop new methods and devices to increase the speed of the web through the drying section.
Another aspect of paper machine technology is that the quality requirements imposed on the paper produced, in particular on fine paper and copying paper, are even now quite strict, and will become even more demanding. Particularly high requirements are imposed on the symmetry of the paper in the z-direction and on the properties of both sides of the face of the paper web as well as on the stability of the paper structure as the paper is heated rapidly in a copying or printing process. These quality requirements of a paper product impose particularly high requirements on the dryer section of a paper machine. However, these requirements are ever more difficult to meet with increasing web running speeds.
In the prior art, a twin-wire draw and a single-wire draw are used in multi-cylinder dryers of paper machines. In a twin-wire draw, the groups of drying cylinders have two wires which press the web, one from above and the other one from below, against the heated cylinder faces of the drying cylinders. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws. These free draws are susceptible to fluttering, possibly causing undesirable web breaks. For this reason, in recent years, increasing use has been made of the single-wire draw, in which there is only one drying wire in each group of drying cylinders. In the single-wire draw type of drying group, the web runs through the whole group on support of the drying wire so that the drying wire presses the web against the heated cylinder faces of the drying cylinders. The web remains at the side of the outside curve on the leading cylinders between the drying cylinders. Thus, in a single-wire draw, the drying cylinders are arranged outside the wire loop and the leading cylinders are arranged inside the drying wire loop.
In the prior art, in normal groups with a single-wire draw, the heated drying cylinders are usually placed in the upper row and correspondingly, the leading cylinders are placed in the lower row. Typically, the rows are horizontal and parallel to one another. Finnish Patent No. 54,627 (corresponding to U.S. Pat. No. 4,202,113, the specification of which is incorporated by reference herein) describes an arrangement wherein normal single-wire groups and so-called inverted single-wire groups are arranged one after the other. In the inverted groups, the heated drying cylinders are placed in the lower row and the leading suction cylinders or rolls are placed in the upper row so that the principal objective, i.e., drying the web symmetrically from both of its sides, is accomplished.
Other references (of Beloit Corp.) also describe a dryer section that includes normal and inverted cylinder groups. In this regard, reference is made to published International Patent Applications WO 88/06204 and WO 88/06205.
With further respect to the prior art, reference is made to U.S. Pat. No. 2,537,129 wherein FIG. 4 illustrates an inverted cylinder group followed by a drying module consisting of a single Yankee cylinder.
In the following, the terms "normal (drying) group" and "inverted (drying) group" are used and generally denote cylinder groups with a single-wire draw of the type mentioned above.
In the prior art, with the use of a single-wire draw in the area of the whole dryer section, various problems have occurred. The present invention is directed to providing novel and efficient solutions to these problems. These problems include the large length of the dryer section which increases the costs of the dryer section and the machine hall. Problems have also arisen from the speed difference between the paper web and the wires. This speed differential results in wear of the wires and, at an extreme, even causes a paper break in the dryer section. The use of an inverted group has also caused problems in the removal of broke which has resulted in increased break times and decreased efficiency. Generally, these problems tend to become worse as the running speeds of paper machines become higher.